1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
Conventional electrophotographic image forming apparatus such as printers, copying machines, and facsimile machines employ electrophotographic processes. A charging roller applies a high voltage to a photoconductive drum to uniformly charge the surface of the photoconductive drum. An exposing unit illuminates the charged surface of the photoconductive drum to form an electrostatic latent image on the photoconductive drum. Then, a developing unit develops the electrostatic latent image into a toner image. The toner image is then transferred onto a print medium, e.g., print paper.
The developing unit can be of non-magnetic one component developing method. This type of developing unit uses a non-magnetic toner. A thin layer of toner is formed on the developing roller that rotates in contact with or in non-contact with a photoconductive drum. The toner on the developing roller is attracted to an electrostatic latent image formed on the photoconductive drum, thereby forming a visible image or toner image on the photoconductive drum.
In such a developing unit, the toner is charged by the use of the friction between the toner particles, the friction between the toner particles and the developing roller, and the friction between toner particles and the developing blade.
With a developing unit in which a developing roller rotates in contact with a photoconductive drum, in order to form a thin layer of toner on the developing roller, the toner-supplying roller rotates in the same direction as the developing roller. The toner is first applied to the toner-supplying roller, which in turn deposits on the developing roller.
FIG. 7 illustrates a conventional image forming apparatus.
Referring to FIG. 7, reference 10 denotes a casing of a developing unit. A photoconductive drum 11 rotates in a direction shown by arrow A. A charging roller 12 rotates in a direction shown by arrow B to uniformly charge the photoconductive drum 11. An LED head 13 illustrates the charged surface of the photoconductive drum 11, thereby dissipating the charges on the photoconductive drum in accordance with an image to be printed. The areas in which the charges are dissipated have a potential of substantially 0 volts.
A toner cartridge 15 is removably attached to the case 10 of the developing unit. A developing roller 17 rotates in contact with the photoconductive drum in a direction shown by arrow C. A toner-supplying roller 16 rotates in contact with the developing roller 17 in a direction shown by arrow D. The toner falls from the toner cartridge 15 into the case 10 of the developing unit, is then supplied by the toner-supplying roller 16 to the developing roller 17, and is finally formed by the developing blade 14 into a thin layer on the developing roller 17.
The toner supplied to the developing roller 17 is deposited to the electrostatic latent image, thereby developing the electrostatic latent image into a toner image.
The toner image on the photoconductive drum 11 is transferred onto recording paper 19 by a transfer roller 18 that rotates in a direction shown by arrow E. After transferring, a cleaning blade scrapes residual toner off the photoconductive drum 11, thereby collecting the residual toner into a waste toner reservoir 15a provided in the toner cartridge 15. An agitator 21 agitates the toner fallen from the toner cartridge 15 into the case 10 of the developing unit, and supplies the toner to the toner-supplying roller 16.
FIG. 8 is a schematic view of a pertinent portion of the conventional image forming apparatus of FIG. 7.
A description will be given of the toner-supplying roller 16, developing roller 17, and developing blade 14.
Referring to FIG. 8, the toner-supplying roller 16 is surface-treated so that the surface of the toner-supplying roller 16 has a plurality of cells. The toner-supplying roller 16 is in contact with the developing roller 17, made of a rubber material, under a predetermined pressure (Japanese Patent Laid-Open No. 2001-242701).
The photoconductive drum 11 rotates at a circumferential speed of 150 mm/s in a direction shown by arrow. The developing roller 17 rotates at a circumferential speed of 192 mm/s in the C direction. The toner-supplying roller 16 rotates at a circumferential speed of 99 mm/s in the D direction.
A metal developing blade 14 has a thickness of 0.08 mm. The tip portion of the developing blade 14 is pressed against the developing roller 17. A power supply E1 applies a voltage of −330 volts to the toner-supplying roller 16. A power supply E2 applies a voltage of −200 volts to the developing roller 17. As the toner-supplying roller 16 rotates, the toner deposited on the toner-supplying roller 16 moves into frictional contact between the toner-supplying roller 16 and the developing roller 17, so that the toner is negatively charged and supplied to the developing roller 17.
The developing blade 14 forms a layer of toner having a uniform thickness on the developing roller 14. Then, the toner on the developing roller 17 is deposited on areas on the photoconductive drum 11 in which the electrostatic latent image is formed, thereby developing the electrostatic latent image with the toner into a toner image.
With the conventional developing unit of FIG. 7, the toner-supplying roller 16 is in pressure contact with the developing roller 17. These two rollers 16 and 17 rotate in the same direction. Therefore, a large torque load is exerted on the toner-supplying roller 16. This also exerts a large torque load on the developing roller 17. Moreover, because the toner-supplying roller 16 is made of sponge, the toner-supplying roller 16 wears easily, and therefore the electrical properties of the toner-supplying roller 16 deteriorate accordingly.
When the toner-supplying roller 16 and developing roller 17 rotate, the frictional force developed between these two rollers causes the toner to wear and agglomerate, resulting in deteriorated electrical properties of the toner.
As a result, the conventional image forming apparatus fails to form an image with high contrast.